1. Field of the Invention
The present invention relates to an optical pickup and an optical data storage apparatus.
2. Description of the Related Art
An optical recording medium such as a magneto-optic disk (MO) or a phase change disk (PD) is used for recording or reading data thereon or therefrom. Also, a read-only compact disc (CD) or laser disk (LD) or DVD-ROM or the like is used for image data or sound data. For these media, an optical data storage apparatus including an optical pickup is used. The optical pickup includes a laser as a light source, a collimating lens and an objective lens. The collimating lens converts divergent light beams emitted from the laser into parallel light beams, and the objective lens converges the collimated light to form a beam spot of small diameter at the optical disk.
For high-density data recording and reading in an optical pickup, the spot diameter of the light beam to be impinged upon the optical disk must be made as small as possible. In order to decrease the spot diameter of light beam, it is advisable to increase the numerical aperture (NA) of the objective lens or to decrease the wavelength of the light emitted from the light source. However, an increase in the numerical aperture (NA) tends to produce aberration depending on inclination of the optical disk. In view of this, it has been considered that for an optical pickup having an objective lens with a large NA, an optical disk (DVD-ROM) whose thickness between its light incidence surface and recording surface is small is more advantageous than a conventional common optical disk (CD) or magneto-optic disk (MO). A decrease in the wavelength of a light beam emitted from the light source produces aberration, depending on the kind of the optical disk.
Under these circumstances, with the improvement of an optical pickup, the types of the optical disks which can be used with an optical pickup are restricted. To this end, an optical pickup which can be used with different types of optical disks is needed.
FIG. 8 in the attached drawings shows an optical pickup disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 7-182690. The optical pickup includes a light-emitting element (laser) 1, a collimating lens 2, a beam splitter 3 for separating light beams, an aberration correction lens 4, an objective lens 5, an optical detection system 6, and a light detector 7. In this arrangement, the aberration correction lens 4 is a concave lens which diverges light travelling toward the objective lens 5, and which increases a convergence distance of the objective lens 5. The aberration correction lens 4 can be moved in a direction indicated by arrow by means of a mechanical moving mechanism (not shown).
If a thin optical disk 8 is used, the aberration correction lens 4 is moved and retracted from the optical path. Light emitted from the light-emitting element 1 is collimated by the collimating lens 2, is transmitted through the beam splitter 3, is converged into a small spot diameter by the objective lens 5, and is made incident on the optical disk 8. The light reflected by the optical disk 8 is passed through the objective lens 5, is reflected by the beam splitter 3, and is received by the light detector 7 through the optical system 6. Thus, data recorded on the optical disk 8 is read.
If a thick optical disk 9 is used, the aberration correction lens 4 is moved into the optical path. The light emitted from the light-emitting element 1 is collimated by the collimating lens 2, is passed through the beam splitter 3 and the aberration correcting lens 4, is converged to form a small diameter spot by the objective lens 5, and is irradiated onto the optical disk 9. The light reflected by the optical disk 9 is passed through the objective lens 5 and the aberration correction lens 4, is reflected by the beam splitter 3, and enters into the light detector 7 through the optical system 6. Thus, data recorded on the optical disk 9 is read.
FIG. 9 in the attached drawings shows an optical pickup disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-204683. The optical pickup includes a first integrated element unit 10, a second integrated element unit 11, a collimating lens 2 disposed behind the first integrated element unit 10, an aperture restriction member 12 disposed at the rear of the second integrated element unit 11, a beam splitter 3 for separating light, and an objective lens 5. In this arrangement, the first and the second integrated element units 10 are 11 and each provided with a light-emitting element 10a or 11a and a light detecting element 10b or 11b.
Light emitted from the first light-emitting element 10a is collimated by the collimating lens 2, is passed through the beam splitter 3, is converged to form a small diameter spot by the objective lens 5, and is irradiated on the optical disk 8. The light reflected by the optical disk 8 is passed through the objective lens 5, is passed through the beam splitter 3, and impinges upon the light detecting element 10b. Thus, data recorded on the optical disk 8 is read.
Light emitted from the second light-emitting element 12 is divergent light which travels while reducing the aperture thereof by the aperture restriction member 12, is reflected by the beam splitter 3, is converged to form a small diameter spot by the objective lens 5, and is irradiated on the optical disk 9. The light reflected by the optical disk 9 is passed through the objective lens 5, is reflected by the beam splitter 3, and impinges upon the light detecting element 11b. Thus, data recorded on the optical disk 9 is read.
In the examples described above, the optical pickup has a single objective lens 5 and corrects aberration for optical disks having different thicknesses. Therefore, the optical pickup is less expensive and more useful than a solution using two different optical pickups for optical disks having different thicknesses. Nevertheless, an optical pickup having the aberration correction lens 4, as shown in FIG. 8, requires a moving mechanism for moving the aberration correction lens 4, thus leading to a large, expensive device. In an optical pickup composed of two light sources and a single objective lens as shown in FIG. 9, since divergent light is used in the optical path including the second light-emitting element 11a, the distance between lenses is strictly limited. In addition, since each beam of light emitted from the light sources is made incident upon the light separating element at different incident angles, the problem that a reflectance property or the wavefront aberration becomes worse due to the light separating element has arisen.
Furthermore, the integrated element units 10, 11 shown in FIGS. 10 and 11, having light-emitting elements 10a, 11a and light detecting elements 10b and 11b incorporated therein, are available on the market. The NA(.alpha.) for the integrated elements 10 and 11 is specified to obtain an intended performance thereof. As shown in FIG. 10, if the collimating lens 2 is used, the specified NA(.alpha.) can be easily satisfied. However, as shown in FIG. 11, if divergent light is used without using the collimating lens 2, it is difficult to provide the specified NA(.alpha.).